Mode changer



NOV. 25, R. M. FANO MODE CHANGER Filed Oct. 5, 1945 2 SHEETS--SHEET 1 FlG.l FIG2 FIG.3

INVENTOR. ROBERTO M. FANO W4 Q, ATTORNEY.

R. M. FANO MODE CHANGER Nov. 25, 1952 2 Sl'IEETS--Sl-IEET 2 Filed Oct.5, 1945 INVENTOR. Fflflf70 M FA /Va 601114 Patented Nov. 25, 1952 MODECHANGER Roberto M. Fano, Boston, Mass, assignor, by mesne assignments,to the United States of America as represented by the Secretary of WarApplication October 3, 1945, Serial No. 620,127

Claims. (Cl. 178-44) This invention relates to transmission lines andmore particularly to inclosed transmission lines such as hollow-pipewave guides and coaxial transmission lines.

It frequently becomes desirable to propagate electromagnetic energythrough an inclosed transmission line which is circularly symmetrical;that is, symmetrical about its longitudinal axis so that any crosssection of the transmission line exhibits circular conducting lines.More briefly, by a circularly symmetrical inclosed transmission line ismeant either a circular wave guide or a concentric coaxial cable theinterior of which substantially is closed from the outside atmosphere.In cross-section these display a circle as the inner part of the outerconductor and a concentric inner circle as the outer part of the innerconductor if one exists. In some systems wherein such propagation isdesirable, it is also desirable that the mode of propagation be one inwhich the electric vectors assume a substantially uniform, symmetrical,and radial configuration, as will appear more particularly hereinafterin this specification for the hollowpipe wave guide and coaxialtransmission line respectively. Such radial symmetry permits theinclusion of rotating joints in the system for turning an antenna orother purposes, which is insensitive in polarization of the transmittedwaves to the angle through which the rotating joint may be turned.However, when the inclosed transmission line has dimensions permittingthe propagation of two modes the problem arises of causing attenuationof one mode, and propagating the desired mode alone.

Among objects of this invention therefore are to provide means forpropagating electromagnetic energy in a mode with radial symmetricalelectric vectors in an inclosed transmission line of circular symmetry,the dimensions of which would otherwise permit the propagation of othermodes; to provide means for propagating the TMai mode and attenuatingthe 'I'E1,1 mode in circular wave guides, the dimensions of which wouldotherwise permit the propagation of both of those modes; to providemeans for propagating the TEM mode, and suppressing the TE1,1 mode incoaxial cables, the dimensions of which would otherwise permit thepropagation of both these modes; to provide coupling junctions betweeninclosed transmission lines of circular symmetry and wave guides ofrectangular cross-section such that energy transference therebetweenwill be from the TEu,1 mode in the rectangular guide to a modeexhibitingradial symmetry in the inclosed transmission" line of circular'symmetry; to provide a coupling junction between a wave guide ofcircular symmetry and a wave guide of rectangular cross-section suchthat energy transference therebetween will be with the TMo,1 mode in thecircular wave guide; to provide a coupling junction between a coaxialtransmission line of concentric, radially symmetric coaxial cable suchthat the energy transference therebetween will be with the TEM mode inthe coaxial cable.

Other objects, novel features, and advantages of the invention will beapparent from the description contained herein. I

In the drawings I Fig. 1 illustrates the configuration of electricvectors in the dominant or lowest mode, the'TE1,1 mode, in a hollow-pipewave guide of circular cross-section;

Fig. 2 illustrates the configuration of electric vectors in the next tothe lowest, or next dominant mode, the 'IMo,1 mode in such wave guides;

Fig. 3 illustrates the configuration of electric vectors in the dominantmode, which is the TEM mode, in a coaxial transmission line;

Fig. 4 illustrates the configuration of electric vectors in the nextdominant, or next to the lowest, mode, which is the TE1,1 mode, in acoaxial transmission line Fig. 5 is a cross-section of a preferredembodiment of the invention;

Fig. 5A is an orthogonal perspective view of the coupling junction oradapter at the junction of the rectangular wave guide and the coaxialtransmission line shown in Fig. 5 and embodying the present invention;

Fig. 5B is the field configuration in an end of rectangular crosssection of the adapter shown in Fig. 5A excited in the 'IEo,1 mode;

Fig. 6 is a top view in partial cross-section of the embodimentillustrated in Fig. 5;

Fig. 6A is a variation of the embodiment illus trated in Fig. 5;

Fig. 7 is a sectional view of another preferre embodiment of theinvention; and

Fig. 8 is a top view in partial cross-section of the embodimentillustrated in Fig. 7.

Referring now to Fig. 1, there is illustrated the TE1,1 mode in circularwave guide. It is apparent from the illustration that this configurationhas only one axis of symmetry, indicated by dotted line In. Fig. 2illustrates the TMo,1 mode in circular wave guide. vectors of Fig. 2show this mode to have the characteristic herein termed circularsymmetry, and further, the electric vectors are radial, emanat ing fromthe center of the guide. The TE1,1 mode The pattern of the electricisthat variously termed dominant, fundamental, or lowest, because, for anygiven diameter of a circular wave guide, the electromagnetic waves oflowest frequency which can be propagated therein will be propagated inthat mode. To state it another way, it is the mode having the lowestcut-01f frequency, so that lower frequency waves are attenuated only,and not propagated. That mode variously termed the next to the lowest,the next highest, or the next dominant mode is the one having the nexthighest cut-off frequency.

Referring now to Fig. 3, there is illustrated the dominant, or TEM mode,in coaxial cable. This mode has radial, symmetric electric vectors.Further, as is well known, the TEM mcde in coaxial cable has no cut-offfrequency; any frequency, however low, may be propagated within acoaxial cable in the TEM mode. The next higher mode in coaxial cable isthe TE1,1 mode illustrated in Fig. 4, which has a higher cutofffrequency, of course, than the TEM mode.

It-will be observed that the TE'1,1 mode in circular wave guide, and theTE1,1 mode in coaxial cable have certain features in common. The generalconfiguration of the electric vectors with respect to the outerconductor is the same. In particular, each pattern has only one axis ofsymmetry, that indicated by line If! in Fig. 1, and that indicated byline H in Fig.- 4. Further, for a given phasing, the electric vectoralong the axisof symmetry has a specified direction, and there is novectorial component of any electric vector in either Fig. 1 or Fig. 4which is opposite the direction specified thereby.

Likewise, the configurations of the TIVIOJ mode displayedin Fig. 2 andthe TEM mode displayed lnFig. 3 have certain features in common. Bothconfigurations have radial electric vectors (which may extend radiallyinward instead of outward forv adifierent phase condition), and arecircularly symmetrical. A consequence of the circular symmetry displayedby these patterns is that any .two orthogonal axes through the center ofthe figure are alsoaxes of symmetry, in contrast with theTE l modeswherein there is only one axis of symmetry. This difference in symmetryof the TEM mode and TMo,1 mode in coaxial cable and circular wave guiderespectively on the one hand and the TE1,1 modes in coaxial cable andcircular wave guides on the other hand is utilized in the presentinvention to suppress the TE1,1 modes and propagate the TEM or TMO,1modes, by exciting the wave guide of circular symmetry in a mannersymmetrical with two orthogonal axes.

Referring now to Figs. 5 and 6 there is illustrated a preferredembodiment of the invention suitable for achieving only the TEM mode incoaxial cable or the TMo,1 mode in circular wave guides even though thecable or Wave guide is proportioned to permit propagation of both of thetwo lowest modes therein at the operating frequency. The transmissionline junction or transducer device in Figures 5 and 6 comprises a box atthe junction of a coaxial conductor l5 and a rectangular wave guide 25.The interior of the box is partitioned and compartmented for thecontinuous propagation of electromagnetic energy between the-coaxialconductor I5 and the wave guide 28. The coaxial cable leis excited atits junction with two rectangular wave guides l6 and H Whoselongitudinal axes |9 and 26 respectively are aligned near the junction.Furthermorathe energy in wave guides 16 and IT,

respectivelyJs in phase at equal distances from the ;center l8 ofcoaxial cable 15 measured along axes I 9 and 20. Aligned axes l9 and 20projected toward each other intersect axis l8 at right angles, and aline 2| perpendicular to axes I9 and 2G and I8 through theirintersection is thereby uniquely determined. Since energy is in phase inwave guides l6 and I! at equal distances from axis I9 and wave guides I6and I? are of rectangular cross section and excited in the TEO,1 modewith electrical vectors perpendicular to the plane of Fig. 6, so thattheir amplitude distribution will be a half sinusoid along any line inwave guides l5 and H perpendicular to axis H3 or 2!], it is clear thataligned axes l9 and 29 and axis 2| form orthogonal axes of symmetry forthe mode generated in transmission line I5.

To assure the in-phase condition required along aligned axes I9 and 29of wave guides it and I7, these may communicate through bends or turns22 and 23 at the ends of the septum or partition 3d away from axis i 3with wave guides 24 and 25 respectively. Wave guides 24 and 25 are alsoof equal lengths and excited by a common source in a symmetrical fashionso that each absorbs theoretically half the energy therefrom. In thisinstance, a rectangular wave guide 26 is used as a common source; fromit energy is fed through two windows 2'! and 28 proportioned forimpedance matching purposes. Different shaped windows or other impedancematching devices may be utilized, however, as the case demands. Forexample, an adjustable terminating plunger, aligned with thelongitudinal axis of wave guide 25 or one perpendicular thereto andextending opposite axis |8 of wave guide l5 may be utilized.

Fig. 6A illustrates another embodiment of the invention in which waveguides fli' and H are excited by forming them as an extension ofcurvilinear wave guides 25 and 25' communicating with a common sourcewave guide such as 26. In order for excitation of wave guide |5' to besymmetrical with respect to two mutually perpendicular axes in a crosssection thereof, the longitudinal axes I9 and 20 of wave guides I5 andI! are aligned inthe neighborhood where they join with wave guide l5 andthese axes pass through axis l3. As before, aligned axes l9 and 2D andaxis 2| perpendicular thereto at the intersection with axis l8 are twoperpendicular'axes of symmetry for the excitation of wave guide !5 whichis thereby excited in the TEM mode. Impedance matching windows may beused in guides 26 or 26'.

Referring again to Figs. 5 and 6, energy propagated though rectangularwave guide 26 may be radiated through windows 21 and 28, formed byprojections 21 and 23 to provide a good impedance match, into guides 24and 25 which are so dimensioned that only the TEo,1 mode may bepropagated therein, with electric vectors parallel to the plane of Fig.5 and perpendicular to that of Fig. 6. propagate only the TEo,1 mode atthe contemplated frequencies. Therefore, excitation of coaxial cable !5will be symmetrical with respect to two orthogonal axes. First it Willbe symmetrical with respect to axis 2| because the distance throughwhich the energy passes approaching from one side of axis 2|after'leaving common source wave guide 26 is equal to the lengthtraveled in approaching the other side of axis 2|. Second, it will besymmetrical with, respect to aligned axes-l9 and 29 because thepropagation of energy-through Waveguides I6 and I1 is sym! metrical withthis axis (and likewise the. plane Likewise wave guides l6 and I1 canvthrough Which'the view of Fig. 5 is taken). Because of the symmetryandthe equal phasing on each side of axis 2| due to equal path lengthswhich the energy has traveled, it is impossible for'an electric field toresult which has a configuration of the TE1,1 mode. Thus, it isimpossible for the TE1,1 mode to be excited, even thou h the radii b anda are such as to permit propagation thereof. The base 29 provided forcentral conductor 30 of wave guide I5 is for impedance 1 matchingpurposes. If inclosed transmission line I5 is of the hollow wave guidetype, central conductor 30 is lacking. A base such as 29 may beprovided, or other impedance matching means may be utilized therewith.Base 29 may have a different shape, to which the outer walls mayconform, as indicated by dotted lines A.

Referring now to Figs. 7 and 8, there is illustrated another preferredembodiment of the invention. A rectangular wave guide 40 forms ajunction with an inclosed transmission line M which is illustrated as acoaxial cable having a central conductor 42. If energy is propagatedthrough wave guide 40, all modes applicable to transmission line 4| willbe generated, at the junction. However, it is presumed that thedimensions of transmission line 4| are such that the dominant and nextdominant modes may be propagated therein, other modes bein subject toattenuation only.

A septum of parts 43 and 43 made of conductive substance and preferablythin is inserted diametrically in line 4|. It is substantially in theplane defined by the axes of the two transmission lines near thejunction. Excitation in wave guide 40 is symmetrical with respect toaxis 44 (which lies in the plane along which the view of Fig. '7 istaken). Therefore, this plane is one of symmetry which is not disturbedby septum 43-43. In fact, the septum may be considered as forming twosemi-annular wave guides having a dominant mode with a cut-off frequencythe same as the cut-off frequency of the TE1,1 mode in a coaxial line ofthe same dimensions if the partition is infinitely thin. The electriclines of the TE1,1 modes are normal to a diametric plane (as c of Fig.4) so that this plane can be materialized in a thin conducting septum orwall without disturbing an existing field of TE1,1 configuration,because the new boundary conditions are automatically satisfied sincethere is no tangential component of electric field at the septum. V

- Since they are equally excited, waves propagated 'insemi-circularguides 4| and 4|" will be equal in amplitude. As explained, they will bealike in configuration and phase, so that the vectors all extend in ageneral way outwards or inwards, as illustrated by the arrowed lines inFig. 8. Therefore, the electric field at the input of coaxial line 4|where septum 43-43 termi- ..--nates will have two-plane symmetry, onealong the plane of Fig. '7 and another perpendicular thereto through theaxis of line 4|. Thus, no TE1,1 mode will be generated in line 4!. Thesections of semi-annular guides 4| and 4| "must be sufficiently long toattenuate to a negligible amplitude higher modes, that is the localfield, generated at the junction With the rectangular guide. A localfield is also present at the termination of septum 43-43 into thecoaxial line.

proper. Therefore, a good circular symmetry of an electrical field isobtained only at a certain distance from the junction. The septum maytaper symmetrically toward the longitudinal axis of cable 4| to improve{impedance match and reduce reflections, and its top portions 45' and 45viewed in the section perpendicular to that of Fig. '7, may be roundedto avoid arc-over if high power carrying capacity is desired. Again, abase 45 is provided for central conductor 42 of line 4| for impedancematching purposes. Also a stub 46 is included beyond the junction forlike purposes. This may be adjustable in length although it is shown asfixed in the drawing. If the principle of'the invention is to be appliedto a circular wave guide instead of coaxial cable 4|, central conductorsuch as 42 is absent, and the septum 43 and 43' may be extendedcompletely" across the wave guide as one member. Thus, the embodimentillustrated in Fig. 8 may be considered as comprising two semi-circularwave guides adapted to feed a circularly sym metrical inclosedtransmission line with twoplane symmetry.

The description herein has been directed to the transfer of energy froma rectangular or semi-circular wave guide to an inclosed trans missionline of circular symmetry. As is well known in the art, a like transferof energy may be accomplished in the reverse direction; that is, theenergy of the desired mode of radial symmetry will be transferred intothe rectangular or semi-circular wave guides with substantially notransfer of energy between other modes. It is not absolutely necessarythat a rectangular wave guide be used as shown in the preferredembodiments.

It will be apparent to those skilled in the'art that there are manyvariations of the invention which do not depart from its scope andspirit,

What is claimed is:

1. A transducer comprising an inclosed substantially circulartransmission line, a first and a sec-0nd straight rectangular wave guidecommunicating therewith and axially opening into each other at one endand closed at their opposite ends, said two wave guides forming withsaid transmission line a common junction in the neighborhood of whichsaid transmission line has circular symmetry, said first and second waveguides being so disposed that their longitudinal axes lie along astraight line that intersects the axis of said transmission line atsubstantially a right angle, a third and fourth wave guide disposedparallel to and having a common wall with said first and second Waveguides respectively, said wall terminating short of the closed ends forcoupling energy from said first wave guide to said third wave guide andfrom said second wave guide to said fourth wave guide, and a fifthrectangular wave guide communicating with said third and fourth waveguides, said third and fourth Wave guides forming with said fifth waveguide a common junction, the longitudinal axes of said third and fourthwave guides lying along a straight line intersecting the longitudinalaxis of said fifth wave guide at substantially right angles wherebythere may be obtained a transference of electromagnetic energypropagated in said transmission line in a mode having radial symmetricalelectric vectors and electromagnetic energy propagated in said fifthwave guide in the TEo,1 mode.

2. A transducer comprising an inclosed substantially circulartransmission line, a first and a second straight rectangular wave guidecommunicating therewith and axially opening into each other at one endand closed at their opposite ends, said two wave guides forming withsaid transmission line a commonjunction in the neighborhood of whichsaid transmission line has circular symmetry, said first and second waveguides being so disposed that their longitudinal axes lie along astraight line that intersects the axis of saidtransmission line atsubstantially a right angle, a third and a fourth wave guide disposedparallel to said first and second Wave guides respectively, a broad wallcommon to said first and third wave guides on opposite sides thereof andcommon to said second and fourth wave guides on opposite sides thereof,said wall terminating short of the closed ends for coupling energy fromsaid first wave guideto said third wave guide and from said second waveguide to said fourth wave guide, and a rectangular transmissionline'communicating with said third and said fourth wave guides, saidthird and fourth wave guides forming with said transmission line acommon junction, the longitudinal axes of said third and fourth waveguides lying along a straight line intersecting the longitudinal axis ofsaid transmission line at substantially a right angle, whereby there maybe obtained a transference of electromagnetic energy propagated in saidfirst transmission line in a mode having radial symmetrical electricvectors into electromagnetic energy propagated in said rectangulartransmission line in the TEo,1 mode.

3. Apparatus in accordance with claim 2 wherein said two transmissionlines are disposed at equal distances from said two common junctions.

4. A transducer comprising an in-closed transmission line, a first and asecond straight rectangular wave guide communicating therewith andaxially opening into each other at one end and closed at their oppositeends, said two wave guides forming with said transmission line a commonjunction in the neighborhood of which said transmission line hascircular symmetry, said first and second wave guides being so disposedthat their longitudinal axes lie along a straight line that intersectsthe axis of said transmission line at substantially a right angle. athird and a fourth wave guide disposed parallel to said first and secondwave guides respectively, a fifth rectangular wave guide communicatingwith said third and said fourth wave guides, said third and fourth waveguides forming with said fifth wave guide a common junction, thelongitudinal axes of said third and fourth wave guides lying along astraight line intersecting the longitudinal axis of said fi-fth waveguide at substantially a right angle, the point of intersection of saidline and said axis lying on a line colinear agated in said fifth waveguide at the TEo.1,

mode.

5. An adapter at the junction of a coaxial line and a rectangular waveguide, comprising a coaxia'l line having an axis and an outerconductorand an inner conductor, a first pair of coaxial rectangular wave guidesopening into each other and into said coaxial line at their inner endsand closed at their outer ends and having aligned axes incident to andin angular relation with the axis of said coaxial line, a second pair ofcoaxial rectangular wave guides parallel to .said first pair of waveguides and opening into each other and into said rectangular wave guideat their inner ends and closed at their outer ends and having alignedaxes incident to and in angul-ar relation with the axis of saidrectangular wave guide to comprise a rectangular adapter to the adjacentouter walls of which adjacent ends of said coaxial line outer conductorand said rectangular wave guide are attached, a partition shorter thanthe inner length of and supported within said adapter and against whichan end of said coaxial line inner conductor is positioned, saidpartition constituting one wall of each of said pairs of coaxialrectangular wave guides, and said rectangular wave guide having an axisincident to and in angular relation with the axes of said coaxial lineand said second pair of coaxial wave guides.

ROBERTO M. FANO.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,129,669 Bowen Sept. 13, 19382,396,044 Fox Mar. 5, 1946 2,401,751 Friis June 11, 1946 2,408,033 BeckSept. 24, 1946 2,410,840 Samuel Nov. 12, 1946

